The present invention relates to a feed unit for moving any desired parts over short distances, such as for moving wire clamps or wire grippers on wire bonders and, more particularly, for feeding the bonding wire by means of wire clamps or wire grippers on ultrasonic wire bonders.
In ultrasonic wire bonding, for which aluminum wire is generally used, each contact (bond) on the bond islands (bond pads) of a semiconductor chip or a substrate is produced using a wedge bonding tool (wedge), whose technical construction is generally known. A number of steps are required to implement a complete bonding cycle, e.g., to produce a wire jumper between the bond pads. To make it possible to perform these steps, the wedge is provided with a wire gripper, also known as a filament clamp, by means of which the bonding wire can, depending on the current process step, either be held in place or advanced toward the wedge or moved away from the wedge in order to sever the bonding wire from the second contact.
During this process, it is necessary prior to forming the first bond of each wire jumper to initially advance the bonding wire under the working surface of the wedge, hence to produce a tail that facilitates execution of the first bond. After forming the bond on the first contact, the wire jumper is pulled to the second contact in the form of a loop, and the free end of the bonding wire is broken off in a defined manner after execution of the second bond.
The required tail length itself is a function of the geometric relationships of the wedge and the bond pad. If the tail is too long, the danger of short circuits to neighboring bond pads exists. If, on the other hand, the tail is too short, a faulty bond will be produced under certain circumstances.
While the wire jumper is being pulled to the second bond contact, the wire gripper must release the bond wire so that the wire can be drawn from a supply spool. After reaching or slightly overshooting the second bond point, the bonding wire must be held fast again so that the necessary loop can be laid and the bonding wire can be broken off after forming of the second bond. While the length of the break-off stroke after the second bond is not critical for the bond itself, it is a contributing factor in determining the overall cycle time.
Prior art drives for wire grippers can be divided into two basic categories, namely, so-called mechanically contacting drives, such as cam-type drives and the like, on the one hand, and on the other hand, non-contacting drives, such as magnetic drives, in which the wire grippers themselves must be supported by suitable guides such as linear guides.
Mechanically contacting drives are characterized by long possible actuating distances at relatively low actuating speeds and high maintenance costs as a consequence of the unavoidable wear.
Moving-coil drives and voice-coil drives come into consideration as examples of non-contacting drives. However, as a result of the limitations on space in the vicinity of the bonding head, it is necessary for such drives to be as compact as possible. This circumstance has the result that these drives cannot be made very mechanically stiff; as a result, they can easily be deflected from their initial position during rapid movements of the bonding head such as those which are encountered in wire bonders with rotatable bonding heads. This deflection can then lead to errors in subsequent movements and also to positioning problems. A further disadvantage of drives of this type is that, when constructed in compact form, they can produce only small actuating forces. Moreover, with such drives it is necessary to work with rigid stops, which are subject to significant wear and also results in the need for the rigid stops to be made adjustable, which in turn significantly increases the adjustment and maintenance cost.
Such a magnetic drive is known, for example, from WO 98/24583 A1. The drive and location positioning of the wire gripper here are performed bidirectionally from a zero position against the spring force of leaf springs with the aid of a linear drive that is connected to program control of the wire bonder and takes the form of a moving-coil drive, a linear motor, or a piezo drive that directly couples the moving elements of the straight-line mechanism to one another.
A previously known piezoelectrically-actuated drive and adjustment element, as described in U.S. Pat. No. 5,900,691, has two piezoelectric stack translators arranged adjacent to one another, the lower end of each being connected to a leg by a solid joint. The upper ends of the stack translators are attached to a rigid traverse bar that is in turn connected to an upper leg by a solid joint. The two legs are connected to one another to form a solid parallelogram.
When the two stack translators are subjected to different electric potentials, a lateral deflection of the solid parallelogram is accomplished. The size of the lateral deflection is determined by the applied electric potential and the potential difference between the two stack translators.
A drive or adjustment element of this type is of very complex construction and hence is expensive to manufacture.
An object of the present invention is to provide a feed unit that is economical to manufacture, ensures adequate system stiffness, and facilitates precise and rapid electrically programmable positioning of the component to be moved.
The foregoing object is attained, in accordance with the present invention, by a feed unit for moving parts over short distances that has a four-bar chain forming a translating solid parallelogram having two parallel rigid opposite legs, one of which constitutes a base leg, connected to one another by elastic elements. A piezoelectric stack translator extends between and is flexibly connected to the rigid legs for articulation relative to the rigid legs and is oriented at an angle of incidence (xcex1) with respect to a base leg that is less than 90xc2x0 and greater than 45xc2x0.
A feed unit according to the invention can be cost-effectively manufactured and has a high system stiffness even at small sizes, which is achieved, in particular, through the use of the piezoelectric stack translator in combination with the solid parallelogram.
With a drive unit of this type, any desired position within the actuating range can be preset as the zero or starting position through electrical actuating variables. Furthermore, all stops can be eliminated with a drive unit of this type.
To avoid parts that move relative to one another and thus cause wear, it is advantageous for the stack translator to be connected in the vicinity of its ends to the solid parallelogram by solid joints.
In order to keep assembly and manufacturing costs to a minimum, the solid joints are joined to the solid parallelogram as a single part and in addition each has one mounting element for the stack translator.
In a refinement of the invention, the stack translator is frictionally clamped between the mounting elements. A glued connection can additionally be provided.
The solid parallelogram is further characterized in that the rigid parallel legs are connected to one another by elastic elements that are designed as solid elastic elements, preferably formed by recesses in the solid parallelogram which form flexible couplings.
The lowest manufacturing and assembly cost is achieved when the solid parallelogram is constructed as a single piece, which can be accomplished by machining the solid parallelogram from a metal plate by milling or erosion.
In one variant, the solid parallelogram can also be embodied as multiple pieces, where leaf springs or leaf-spring-like elements are arranged between the rigid legs as the elastic elements. A further advantageous refinement of the invention is characterized in that a wire clamp is attached directly to a rigid leg of the translating solid parallelogram and that the other rigid leg is attached to the bonding head.
Rapid and precise positioning of the wire clamp or other components with high actuating force can be implemented with the feed unit in accordance with the invention, where positioning in a simple manner is possible through the provision of electrical actuating variables. In combination with an electrical control loop, position control of the wire clamp can be implemented easily and without the need for troublesome solid stops.
This has the particular advantage that alteration of the tail length and the break-off stroke can now be accomplished exclusively by programmatic means and the time-consuming adjustment steps previously required can be completely eliminated. A significant improvement in the operational properties of the bonder is thus achieved by simple technical means, since working on solid stops is avoided. This means that the time for setting up the system that is otherwise necessary after moving into stops can be saved, which results in further advantages.
This means that all bond parameters that are affected by the wire clamp and its movement can now be predetermined by the feed unit in accordance with the invention by means of program control of the bonder.